A varistor (also known as a non-linear resistor) has nonlinear electrical properties--in particular, it exhibits a nonlinear voltage-current behavior. Below a critical voltage (also variously referred to as the breakdown voltage, the switching voltage, or the threshold voltage) a varistor is highly resistive, in the megohm range, and acts essentially as an insulator, allowing only a small leakage current to pass through it. When the breakdown voltage is exceeded, the resistance of the varistor decreases dramatically, and the varistor conducts substantial amounts of current--i.e., acts as a conductor. The voltage-current relationship of a varistor is described by the equation EQU I=(V/C).sup..alpha.
where I is the current flowing through the varistor; V is the voltage across the varistor; C is a constant which is a function of the dimensions, composition, and method of fabrication of the varistor; and .alpha. (alpha) is a constant which is a measure of the nonlinearity of the varistor. A large .alpha., signifying a large degree of nonlinearity, is desirable. High quality varistors typically have an .alpha. greater than 20, as high as 50 or above.
One common application for a varistor is a surge arrester. In a surge arrester, the varistor is connected in series between an electrical system and ground. At ordinary system voltages, the varistor is highly resistive, so only a leakage current flows between the system and ground. If there is a sudden surge in the system voltage, exceeding the breakdown voltage (for example because of a lightning strike), the varistor becomes conductive and shunts the excess current to ground. This way, the system voltage is prevented from exceeding a predetermined maximum voltage above which damage to system components could occur. Systems so protected by varistors can range in size from a power distribution network to an individual electronic device, such as a computer, a television set, and the like.
Another application for a varistor is as an element for controlling the switching of pixels of liquid crystal displays. See, for example, Raychem, WO 92/18972 (1992).
The size of the varistor element in a surge arrester varies in accordance with the size of the system protected and the desired switching voltage. In particular, the switching voltage is directly related to the thickness of the varistor element across which the current is to pass. For some systems, the varistor element may be quite small, for example a chip only tenths of millimeters thick and only several millimeters wide and long. One way to produce small varistor chips is to make a tape of varistor material and dice it into appropriately sized chips. However, because of variations in the thickness of varistor tape, the thickness of the varistor chips and hence their switching voltage are subject to undesirable variations from chip to chip. Thus, a method of reliably making varistor chips of known dimensions and switching characteristics is desirable.